This invention concerns the use of marks to identify and/or authenticate documents. More particularly, the invention concerns the identification and/or authentication of a document by means of a mark on the document that is separate from information that the document contains.
A document contains information. According to an authoritative definition, a document is xe2x80x9c . . . information and the medium on which it is recorded . . .xe2x80x9d. In this regard, a document can be embodied in an image on a piece of paper, written information on a print-supporting medium, and electronic or optical data on a storage medium. Examples of common documents abound. Checks, photographs, movies on film or video, audiotapes, CD""s, and passports are examples of documents.
It is increasingly important to be able to identify, authenticate and/or validate documents. In the past, such functions have been provided, for example, by the xe2x80x9cchopxe2x80x9d on a sheet of calligraphy, the account number on a check, a photograph on a passport, and a signature or thumbprint on a testament. The purpose of such measures is to prevent the illegal, unauthorized, unscrupulous, or nefarious use of original documents and their authorized copies. Consider, for example, the unauthorized use of a counter check that identifies a checking account depositor correctly. Without a discernable account number, the check will not be authorized for payment. However, the depositor will be assured of clearance of a check that bears both her account number and signature.
In the modern world of digital information, the ease with which documents can be obtained, copied, modified and transferred necessitates the provision of corresponding means for document identification, authentication and/or validation.
In this application, the term xe2x80x9cdigital markxe2x80x9d(or, simply xe2x80x9cmarkxe2x80x9d) is used to signify the existence of a digital object that may be appended, added to or placed on a document for the purposes of identifying, authenticating, or otherwise validating the document. The digital object embodying the digital mark is typically derived from signals representing information beyond that which is apparent in the document. In this regard the term xe2x80x9cdigital objectxe2x80x9d signifies a perceptible or discernable object that is, or is created or derived from a digital representation that may be a vector, array, or sequence of ones and zeroes, or of pixels. Once created or derived, the mark may be appended to the information in the document, separately from the information, or may be embedded in the information so as to make it difficult to perceive when the document""s information is comprehended in a content-appropriate manner. Thus, for example, an audiotape may have a digital mark woven into the audio information in such a way as to be imperceptible to a listener, but tractable to authenticating means that knows how and where to find the mark.
Digital marks that are perceptible and separate from the information in the document which they identify, authenticate and/or validate have the advantage of being relatively simple and inexpensive to locate and to process. No special means are necessary to perceive and extract the mark from the information contained in a document. Decoding the mark is simply a matter of applying a process that is inverse to that utilized for generating the mark. Any document without the mark will be presumed to be unidentifiable, inauthentic, or otherwise invalid.
The invention provides derivation of a digital mark that is to be placed in a document apart from the information that the document contains. The invention is based on the critical realization that a robust mark may be derived by a process that combines the information in the document with private, extra-documentary information. The process receives the private information as an input and then scrambles the private information in response to the information in the document. Scrambling is a process of pseudo-randomization of the input private information that may be accomplished, for example, by means of a linear feedback shift register (LFSR) clocked in response to the document information. The scrambled private information provides a digital mark that may be placed in or on the document, apart from the information content of the document. In a particularly useful embodiment, the digital mark is processed to create a xe2x80x9cblazexe2x80x9d by image processing that is analogous to xe2x80x9csmearingxe2x80x9d the pixels of an image.
Accordingly, it is an objective of this invention to cause the generation of a digital mark by scrambling private information in response to the information of a document that is to be marked.
In this regard, the scrambling is particularly efficiently accomplished by seeding an LFSR with the private information and then clocking the operation of the LFSR in response to the document information.
These objectives and other advantages become evident in when following detailed description is read with reference to the below-described drawings.